In recent years, attention has been focused on the use as next-generation nonvolatile ferroelectric memory materials of single crystal thin films of multi-component oxides of which Bi (bismuth) is a constituent element. One reason is that a multi-component oxide which contains Bi as a constituent element has generally a large spontaneous dielectric polarization and its dielectric properties deteriorate quite a little with repeated polarization reversals. Another reason is that a multi-component oxide which contains Bi as a constituent element has generally a crystal structure of perovskite type which is easy to epitaxially grow on many substances, and is optimum for use as a thin film material for nonvolatile ferroelectric memories having a degree of integration on the level of a VLSI (very large scale integrated circuit) since an epitaxially grown thin film is low in defect.
Of multi-component oxides of which Bi is a constituent element, especially Bi4Ti3O12 being excellently large in the magnitude of spontaneous polarization and easy in its single-crystallization in the form of a thin film has vigorously been studied as to its polarization mechanism and methods of making its single-crystal thin film which is low in defect (See: Takeshi Kijima, et al., Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 127-130 and X. Q. Pan, et al., APPLIED PHYSICS LETTERS Vol. 83, No. 12, 22 Sep. 2003, pp. 2315-2317.).
By the way, the use of a thin film as a material for a highly integrated device as mentioned above requires the thin film to be low in the density of defects such as dislocations and grain boundaries and to have an atomically flat surface, i. e., to be high in crystallinity or crystalline perfection. Accordingly, techniques have so far been studied of making a thin film higher in crystalline perfection, using a vapor phase growth method such as CVD (chemical vapor deposition), laser ablation or sputtering process.
In the vapor phase growth methods, however, which are essentially growth methods in a non-equilibrium state, it is difficult to eliminate defects such as grain boundaries and dislocations completely and due to these defects it is hard to obtain an atomically flat surface.
The present inventors have developed the Tri-Phase Epitaxy which is designed to effect crystal growth in a state that a solid, a liquid and a gas phase coexist (See Journal of the Metallurgical Society of Japan Vol. 66, No. 4 (2002), pp. 284-288.). This method is a method in which a seed layer is built up on a substrate surface and has a layer of a substance (hereinafter referred to as “flux layer”) built thereon which is capable of producing a eutectic crystal with an objective substance but not producing any compound with the objective substance, and the objective substance is fed by a vapor phase growth method onto the flux layer heated at a temperature not less than the eutectic point to epitaxially grow. In this method, the objective substance in the flux layer comes to be in a liquid phase from which the objective substance precipitates on the seed layer to epitaxially grow. Being essentially a liquid phase epitaxy and thus close to crystal growth from a thermal equilibrium state, this method can grow on a substrate a thin film which though being a thin film has a crystalline perfection comparable with that of a bulk crystal. Using this method, the present inventors have also succeeded in the making of a multi-component oxide containing Bi as a constituent element, which though in the form of a thin film has a crystalline perfection that can compare with that of a bulk crystal (See JP 2005-1987 A).